For various reasons it may be necessary to change the speed of rotation of an output shaft or driven shaft vis-a-vis an input shaft or driving shaft and, especially, to provide speed reduction and torque multiplication between a driving shaft and a driven shaft or, conversely, to provide speed multiplication.
Numerous transmission systems have been provided for this purpose and those which are of interest here are high-performance transmissions or reducers, i.e. transmissions of the type which are useful in manipulating robots (mechanical arms or the like, for example, machining and delicate or high-load applications), indexing heads and dividing heads for use in numerical control technology, for use with special propulsion engines for monitoring or vehicles or controls in nuclear power plants or in conjunction with nuclear transmission cores, generally, for hoists or for other specific devices.
In all of these applications it is important that the speed reducer be of comparatively small size and low weight, have a high torque-transmitting capacity, be free from play and provide precise transmission of force with high-ratio reduction.
With existing reducers, these articles are for the most part mutually incompatible and attempts to eliminate play by improving fabrication tolerances and attempts to reduce size and weight by improvement of the alloys or materials used have not eliminated problems.
In the prior art system, a flexible bell having a toothed periphery is received in a rigid ring having internal toothing and is deformed against the ring by a device rotating within the bell. This device assures contact between the bell and the ring at two opposite points. With each rotation of this device, the bell is offset with respect to the surrounding ring by an angle equal to the distance between the number of teeth between the interengaging crowns of the bell and ring. While such a reducer is advantageous because it provides significant speed reduction in a single stage and is reversible so that it can also be used as a multiplier, it can be fabricated only at comparatively high cost, is bulky because it requires a bell of sufficient length that its deformation is effected with a suitable geometry for the contact between the teeth and is not free from play.
In another prior art device, a rigid crown is fixed to the transmission casing and is formed at one of its surfaces perpendicular to its axis with annular contact surfaces for engagement with a elastically deformed crown centered on the same axis and rotatably fixed to the slow shaft. This deformable crown has an annular contact surface for engagement with the surface of the first crown and a length greater than the length of the contact surface of the latter. The rapid shaft is provided with means ensuring contact at least one point between the rigid and flexible crowns. This means is in the form of a cage containing balls disposed diameterically opposite one another across the axis and which press the flexible crown at two diametrically opposite points against the rigid crown.
While the balls are effective to ensure the pressure of the flexible crown against the rigid crown and hence to generate an annular displacement in the flexible crown which is a fraction of the annular displacement of the rapid crown, this device does not admit of a well defined interengagement of the active surfaces, is not able to take up effectively the axial forces which are required and, in general, does not satisfy the requirements outlined above except for the requirement that a high ratio be provided.